Game Help: When people sell good pokemon

So I'm pretty oblivious when it comes to the inner workings of pokemon breeding, trading, and game hacks and tricks these days. Here is my question. I see people selling pokemon (as trade offers) on ebay all the time. The weird thing is, it's usually pokemon that are rare and ones that you can only receive once in your game like legendaries. I even see old event pokemon with specific Original Trainers. How?? How can a single person have such a surplus of legendaries, shinies, legendary shinies, etc? You can't breed most of the rare ones, so are they just hacking the game? And if so, wouldn't that mean that the pokemon they are sending you is not legit and therefore wouldn't work with Bank anyways?

Either hacking altogether, or cloning of existing Pokémon. I wouldn't trust it. :x

Do they come in bulk? It has to be what the user above me said. I'll have a some legendaries soon from legit cartridges but those are getting more and more by the day here.

Yeah, I want to avoid filling my dex with any pokemon that I didn't catch myself or get from a fair trade, but some of them seem near impossible to get ahold of. I tried to contact one of the sellers so they could explain to me how they got their pokemon in the first place, but so far no response. So that must mean it's not through legit means and they don't want to admit it.

Yes all Pokemon are hacked into the game when its too good to be true. Legendary Pokemon are impossible to get shiny, + 6 IV , + The correct nature you wanted. even if you soft reset it is almost impossible to get even one of these traits correct let a lone all 3. Same goes for the fossil Pokemon since they sadly cant be breed :(

I never knew the fossils can't breed! :O

Okay so, none of them are legit. Does this mean they're not tradable between games and not Bank/HOME compatible?